Variant thyroxine-binding globulin in serum of Australian aborigines: a comparison with familial TBG deficiency in Caucasians and American blacks.

About 40% of clinically euthyroid Australian Aborigines have low concentrations of total thyroxine (TT4) and triiodothyronine (TT3) in serum. While the finding of normal concentrations of serum thyrotropin (TSH) in such individuals is compatible with their eumetabolic state, the reason for the finding of a low free T4 index (FT4I) has been unclear. A genetic variant of T4-binding globulin (TBG) with reduced affinity for T4 has been suggested but decrease in the absolute concentration of TBG has also been reported. In this study, we measured various parameters of thyroid function in 20 serum samples from euthyroid Australian Aborigines selected for their low TT4 levels. Results were compared to those obtained in serum samples from Caucasians and American Blacks with inherited partial TBG deficiency, 15 of which were matched to the Aborigines by their TBG and 20 by their TT4 concentrations. Results were also compared with those from another group of 20 samples from Caucasians and American Blacks with normal TBG concentration, matched to the Aborigines by their serum TT4 concentration. TBG in serum from these Australian Aborigines was immunologically identical to that in Caucasians and American Blacks in terms of parallelism of serially diluted samples in the TBG radioimmunoassay (RIA).(ABSTRACT TRUNCATED AT 250 WORDS)